1. Field of the Invention
The present invention relates to a tire-state detection device, and more particularly to a tire-state detection device for transmitting detection results using radio signals to an external destination, the device used by being fitted to a rim inside a tire.
2. Background Information
Generally, a tire-state detection device that is equipped with a wireless device for transmitting air-pressure data is fitted to a rim well in a system for monitoring air pressure inside a tire as described in Japanese Unexamined Patent Publication 2006-327460. Characteristics often greatly fluctuate because an antenna equipped in the tire-state detection device is affected by nearby metallic parts, such as the rim. Therefore, antenna designs have been required for providing tire-state detection devices on metallic parts, such as the rim and the like. However, rim shapes are varied, and the resonance frequency and impedance of the antenna fluctuate dramatically if the distance between the antenna and the metal constituting the rim are different due to the shape of the rim.
For example, if a tire-state detection device that is equipped with a transmitter capable of a transmitting frequency of 315 MHz, and an antenna having a resonance frequency of 315 MHz is optimally fitted in the rim, a good impedance characteristic will be obtained, as shown in FIG. 26. However, if the tire-state detection device is fitted slightly away from the rim, or is fitted on a rim having a different shape, antenna resonance frequency will shift dramatically, as shown in FIG. 27.